Modern power supplies are known as "switching regulator power supplies."
In most switching supplies, the 110 volt AC input is first rectified by
two diodes and filtered by a pair of capacitors. This creates two high-
voltage sources; one positive and the other negative.
A pair of transistors is then used to switch these high voltage supplies
across the primary winding of a transformer. This switching action is
very fast. A typical switching speed is around 40,000 cycles per second
or 40KHz. An integrated circuit is commonly used to control the
transistors. This IC not only controls the speed at which the
transistors are switched, but also controls the amount of time that each
transistor is energized. The output voltage of the power supply is
determined by the "on" time of the transistors. If the transistors are
keep on for a longer period of time, the output voltage of the supply
will rise, while shorter times lower the output voltage. This is known
as "pulse-width modulation."
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